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Integrated Stratigraphic Study of the Rhuddanian- Aeronian (Llandovery, Silurian) Boundary Succession at Rheidol Gorge, Wales: a Preliminary Report

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Integrated Stratigraphic Study of the Rhuddanian- Aeronian (Llandovery, Silurian) Boundary Succession at Rheidol Gorge, Wales: a Preliminary Report Integrated stratigraphic study of the Rhuddanian- Aeronian (Llandovery, Silurian) boundary succession at Rheidol Gorge, Wales: a preliminary report OPEN REPORT OR/18/139 The National Grid and other Ordnance Survey data are used BRITISH GEOLOGICAL SURVEY with the permission of the Controller of Her Majesty’s Stationary Office. License No. OPEN REPORT OR/18/139 xxxxxxxxxxxx Keywords Aeronian, Rhuddanian, Llandovery, Silurian, GSSP, Rheidol Gorge, graptolite, Chitinozoa, sedimentary log Front cover Integrated stratigraphic study of the Rheidol Gorge section; note prominent bedding surface (by Rhuddanian-Aeronian (Llandovery, rucksack) 0.8 m below the first appearance of triangulatus Silurian) boundary succession at Rheidol Biozone graptolites. Gorge, Wales: a preliminary report Bibliographical reference: Melchin, M. J., Davies, J. R., De Weirdt, J., Russell, C., Vandenbroucke, T. R. A., Melchin, M. J.1, Davies, J. R.2, De Weirdt, J.3, Russell, C.4, Zalasiewicz, J. A. 2018. Integrated 3 5 stratigraphic study of the Vandenbroucke, T. R. A. , Zalasiewicz, J. A. Rhuddanian-Aeronian (Llandovery, Silurian) boundary succession at 1 Rheidol Gorge, Wales: a Department of Earth Sciences, St. Francis Xavier University, preliminary report. British Geological Survey Open Report, Antigonish, NS, Canada 2 OR/18/039. Department of Geography & Earth Sciences, Aberystwyth Copyright in materials derived University, Aberystwyth SY23 3DB, UK (also British Geological from the British Geological Survey, Cardiff Office, Cardiff University, Main Building, Park Place, Survey’s work is owned by UK Research and Innovation (UKRI) Cardiff, CF10 3AT, Wales, UK) and/or the authority that 3Department of Geology, Ghent University, Krijgslaan 281 S8, BE- commissioned the work. You may not copy or adapt this publication 9000, Ghent, Belgium without first obtaining permission. 4School of Earth and Environment, University of Leeds, Leeds LS2 Contact the BGS Intellectual Property Rights Section, British 9JT, UK Geological Survey, Keyworth, 5Department of Geology, University of Leicester, University Road, e-mail [email protected]. You may quote extracts of a reasonable Leicester LE1 7RH, UK length without prior permission, provided a full acknowledgement is given of the source of the extract. Maps and diagrams in this book use topography based on Ordnance Survey mapping. © UKRI 2018. All rights reserved Keyworth, Nottingham British Geological Survey 2018 Acknowledgements Melchin et al. (2016), at the International Geoscience Programme Project 591 – Closing Meeting ‘The Early to Mid Palaeozoic Revolution’ (Ghent University, 2016), presented the preliminary results of an ongoing study to assess the candidacy of the famous Rheidol Gorge section (mid Wales, UK) as a replacement GSSP for the base Aeronian Stage. A group of conference participants were shown the section as part of the post-conference field trip to the Welsh Basin. This report represents an updated and revised version of the field guide compiled for that visit. The land owners at Bryn-brâs Farm and the George Borrow Hotel are warmly thanked for facilitating vehicular access both for the post-conference visit and during the course of our study. Mark Williams assisted with the field logistics and sample collection and Rosemary Melchin and James Wilkinson also assisted in the collection of the graptolite and isotope samples. Several of the figures in this account bear testimony to the excellent drafting skills of Antony Smith (Aberystwyth University). Support staff at Ghent and Leicester universities are also warmly thanked for their assistance in the preparation of samples. Financial assistance to MJM for this project was provided by a Natural Sciences and Engineering Research Council of Canada Discovery Grant. As an Honorary Research Associate, JRD also acknowledges the support of staff at BGS notably Rhian Kendall (BGS Cardiff Office) for her review of the submitted manuscript and for facilitating its approval and release. Abstract: Rheidol Gorge, approximately 17 km west of Aberystwyth, mid Wales, exposes a continuous succession of strata from the middle part of the upper Rhuddanian Coronograptus cyphus Biozone through the lower Aeronian Demirastrites triangulatus (= Monograptus triangulatus) Biozone. Parts of the Aeronian succession are well known for their beautiful lower Aeronian graptolites preserved as pyrite internal moulds. We measured this section and sampled for graptolites, palynomorphs, and for lithological and geochemical analyses. One of our objectives was to assess the section for its suitability as a candidate for a new Global Stratotype Section and Point for the base of the mid-Llandovery Aeronian Stage. The succession alternates between bioturbated grey mudstones lacking in graptolites and laminated, graptolitic black shales. The black shales commonly show thin, interbedded siltstones. The grey mudstones are interpreted to represent deposition under oxic to dysoxic conditions, the black shales an anoxic seafloor environment. The strata have undergone low- grade metamorphism, commonly with a weakly to moderately developed cleavage, and the graptolites often show ductile and/or brittle deformation. Strata of the middle to upper cyphus Biozone are c 10.2 m thick and yield graptolite faunas of varying diversity and preservation quality. 0.8 m below the base of the D. triangulatus Biozone there is a change from predominantly organic-poor mudrocks with interbeds of darker shales with sparse graptolites to an interval of predominantly black shales with a relatively rich graptolite fauna. The graptolites in the black shale interval, which spans the zonal boundary, are flattened or in partial relief, commonly deformed, and the strata tend to break along cleavage rather than bedding planes. Nevertheless, a distinctive graptolite fauna occurs through the boundary interval that allows good correlation with successions in other parts of the world. The base of the D. triangulatus Biozone is marked by the first appearance of D. triangulatus. Other species first appearing just below the base of the D. triangulatus Biozone that are useful for international correlation include Pristiograptus concinnus and Pseudorthograptus finneyi. Strata rich in well-preserved, pyritic graptolites become common about 2.3 m above the base of the triangulatus Biozone. Chitinozoans are poorly to moderately well preserved in the section and indicate the Spinachitina maennili Biozone through the boundary interval, without any significant faunal changes, as is the case in many other parts of the world. 1. Introduction The first comprehensive geological and palaeontological study of the Rheidol Gorge area was published by Jones (1909). That paper summarized the previous geological studies in the region and thoroughly documented the stratigraphy, structure and graptolite biostratigraphy of the Plynlimon and Ponterwyd areas, including a detailed geological map and description of the Rheidol Gorge section, just south of Ponterwyd (Figs 1 and 2). Jones showed that the Rheidol Gorge section exposes an almost continuous succession of strata through much of the interval that is now known as the Rhuddanian and Aeronian stages. He also documented the succession of graptolite faunas and biozones through the section. Since that time, a number of papers have included material from Rheidol Gorge in paleontological studies (e.g., Elles and Wood, 1901-18; Challinor, 1928, 1945; Packham, 1962; Jones and Rickards, 1967; Cullum and Loydell, 1996; Zalasiewicz et al., 2009). However, the importance of the section was most clearly established, both from a graptolite systematic and biostratigraphic perspective, by the monograph of Sudbury (1958), which provided a detailed analysis of the early Aeronian triangulate monograptids that has since formed the basis for the classification of these taxa, and has also provided a key framework for the regional and global graptolite zonation of the lower Aeronian (e.g., Rickards, 1976; Zalasiewicz et al., 2009). Most recently, Cullum and Loydell (2011) provided a new lithological log of the upper Rhuddanian and lower Aeronian portions of the section and Russell et al. (2013) undertook a very detailed study of the sedimentology of the strata at and just below the Rhuddanian-Aeronian boundary interval. One of the most important points of the Cullum and Loydell (2011) paper was that they showed that graptolites occur at many more stratigraphic levels in the section than had been documented previously, and that graptolites occur throughout the immediate interval that spans the base of the Demirastrites triangulatus (= Monograptus triangulatus) Biozone. In the definition of the Global Stratotype Section and Point for the base of the Aeronian Stage, it is stated “… the marker point for the base is within a continuous lithological section through part of the Trefawr Formation at the base of locality 72 in transect h as described by Cocks and others (1984, p. 165, figs 60, 61, 62, 62). This point correlated with the base of the base of the Monograptus triangulatus graptolite Biozone in the section” (Bassett, 1985). It has since been pointed out by a number of authors (e.g. Melchin et al., 2004, 2012; Davies et al., 2011, 2013), however, that at the GSSP, the D. triangulatus Biozone was only recognized in one sample by the occurrence of one taxon, Monograptus austerus sequens, and that species is elsewhere known only to occur in the middle of the D. triangulatus Biozone. Therefore, the GSSP does not appear to correlate
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